Cell therapy employs a technique in which cells from a living donor are implanted into a subject either directly or after being cultured in vitro. Cells for use in implantation are suspended in a solution suitable for implantation, and adjusted to a suitable concentration prior to implantation. Cells taken from a living donor, optionally cultured in vitro, contain unnecessary tissue-derived components, culture media, and the like, and are typically available as dilutions in particular solutions. Accordingly, the following processes are necessary prior to implantation: removal of unnecessary components and media, and replacement with a solution suitable for implantation (washing); and concentration of cells to a level suitable for implantation. So far, centrifugation has been a principal technique used for such concentration and washing processes.
For example, a technique for separating and concentrating regenerative cells from human tissues by centrifugation is known (Patent Literature 1). Unfortunately, centrifugation is a technique that may be employed in a limited number of facilities because of the necessity of a large-scale device, a burden on cells, and high costs.
By contrast, compact, easy-to-handle devices have been designed for the separation, purification and filtration of cell suspensions using a hollow fiber separation membrane (Patent Literature 2).
Hollow fiber separation membranes proposed so far have various pore sizes, and appropriate sizes are chosen according to the size of target substances and the desired filtration flow rate. For example, hollow fibers with large pore sizes are used to achieve high filtration flow rate. Fibers with too large pore sizes, however, may cause pores to be clogged with proteins or cells present in samples, for example. Patent Literature 3 discloses hollow fiber products designed to overcome this. These products include hollow fibers with relatively large average pore sizes to provide a high filtration flow rate with the aim of high yields, while including in the hollow fibers an anionic resin to prevent clogging in pores. For safety reasons, hollow fibers including two or more materials, however, are not preferred for use in the concentration of cultured cells to be returned to the body because leakage of the materials and the like are likely to occur. Another approach for preventing clogging in pores is to employ certain washing/filtration conditions and the like (Patent Literature 4). Unfortunately, this approach is likely to greatly damage cells because of the necessity of back-washing of a hollow fiber membrane.